ABSTRACT Support is requested for a Keystone Symposia conference entitled Cell Competition in Development and Disease, organized by Drs. Margaret A. Goodell, Laura Johnston and Thomas P. Zwaka. The conference will be held in Tahoe City, California from February 24-28, 2019. Cell competition represents a radical departure from the established view that embryonic development is simply a matter of following a preprogrammed set of rules. Instead, it is a highly conserved process that promotes the context dependent elimination of less fit cells and stimulates growth of more fit cells during growth and homeostasis. Although it has long been known that the basis of competition is the ability of growing cells to monitor fitness of their neighbors, and can be induced via differences in protein production capacity, Myc levels and apico-basal cell polarity, only recently have signaling and effector mechanisms been identified. This conference aims to bring together, for the first time, researchers, from diverse fields, who study competitive and cooperative interactions between cells. It will cover recent findings on quality control systems, developing tissues, stem cell populations, and tumorigenesis, as well as address important evolutionary aspects of competitive and cooperative behavior in diverse model systems. New technologies have uncovered the prevalence of cell competition in humans, with surprising outcomes and implications for human disease. As the first broad meeting of this sort, we expect to define critical questions shared by the diverse investigators and shape this emerging area. Given the wide range of developmental and homeostatic systems that are controlled by cell competition, understanding the mechanisms and consequences of competitive interactions may permit the manipulation of these processes for therapeutic purposes.